Oyster Species of the Sub Tropical Coast of Pakistan (Northern Arabian Sea)
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Indian Journal of Marine Sciences Vol. 31(2), June 2002, pp. 108-118 Oyster species of the sub tropical coast of Pakistan (northern Arabian Sea) Ghazala Siddiqui & Muzammil Ahmed Centre of Excellence in Marine Biology University of Karachi, Karachi-75270, Pakistan [ E-mail: [email protected] ] Received 11 April 2001, revised 1 January 2002 In the present study 9 species of oysters belonging to three genera (Crassostrea, Saccostrea and Ostrea) were identified on the basis of their conchological and malacological features. These included Crassostrea gryphoides, C. madrasensis, C. belcheri, C. glomerata, Saccostrea cucullata, S echinata, Ostrea nomades, O. folium and O. cristagalli. Two species Ostrea nomades and O. cristagalli which are being reported for the first time from Pakistan. Crassostrea gryphoides and C. rivularis from Hub River Delta were earlier treated as separate species have been assigned to a single species C. gryphoides. Crassostrea gryphoides and C. madrasensis, which occur in the same habitat and show marked similarity in their external shell morphology, but differ in the colouration of their adductor muscle scar, are treated here as separate species. [ Key words : Taxonomy, oyster, Crassostrea, Saccostrea, Ostrea ] The coast of Pakistan, which is confined to the north- 1992 to December 1993. Collections from Buleji ern Arabian Sea, lies outside the tropics, as its south- rocky ledge were made from July 1992 to December eastern boundary is only a few miles short of Tropic 1993. From Buleji sampling was also done in April of Cancer. The southeastern extremity touches the 1994. The oysters were also collected from Paradise India with whom Pakistan shares a more or less com- Point, Karachi from January 1992 to May 1992. mon oyster fauna. The collection sites are shown in Fig. 1. Hub Delta The information regarding the taxonomy of Paki- is mostly sandy but the mouth of river forms sandy- stani and Indian oysters is based on the several re- cum-muddy platforms, which are fairly easy to walk ports1-23. From Pakistan, 4-10 species of oysters were recognized and were placed in genus Ostrea24-26. Ahmed27 updated the nomenclature and placed the oysters under two genera, Crassostrea and Ostrea, recognizing six species of Crassostrea, (C. rivularis, C. viriginica, C. gryphoides, C. tuberculata, C. glomerata and C. quercina) and one species of Ostrea, (O. folium). Later C. gryphoides and C. rivu- laris were placed in single species C. rivularis; and C. glomerata and C. tuberculata in the genus Saccos- trea.28 Ahmed et al.29 from Gwadar (Balochistan coast) included the tuberculated oysters like C. tuber- culata and C. echinata in the genus Saccostrea. The present paper reports the presence of nine species of oysters by attempting to update their nomenclature. Materials and Methods Oysters were collected from three localities, i.e, Hub River Delta, Keamari Seawall (Karachi Harbour) Fig. 1 ⎯ Collection sites* of oysters along Sindh and Balochistan and Buleji. From Hub Delta the collections were Coasts. (♣ Crassostrea gryphoides; U C. madrasensis; made from August 1992 to December 1993, from 8 C. belcheri; Ä C. glomerata; ♠ Saccostrea. echinata; ◗ S. Keamari the samples were obtained from October cucullata; ▼ Ostrea nomades; O. folium; ? O. cristagalli). Siddiqui & Ahmed : Oysters of Pakistan 109 on. The delta has been cut off now from the main Ahmed27, Thomson31 and Stenzel32. Other publications river due to the recent construction of Hub Dam. The were also consulted33, 34 for the sake of comparison. seawater now extends two km inland from the Sea. The identifications of the oyster species were based The beds of C. gryphoides and C. madrasensis are on the conchological characters i.e; morphology of now virtually on the verge of disappearance from the shell, its colour and luster, absence or presence of here. The salinity here ranges from 32 to 40 ‰. Buleji denticles and shape, colour and position of adductor rocky ledge is a triangular platform extending into the muscle scar. Some malacological features like pres- open Arabian Sea. It is uneven in profile, with its ence or absence of promyal chamber, colour of the right flank (facing the sea) lying a bit low and facing mantle folds and tentacles of the mantle folds were maximum wave action. In this wave swept part a also examined. The morphological features used for luxuriant growth of seaweeds and zooanthid coelen- the identification of the oyster species are shown in trates was observed. The middle part of this ledge is Fig. 2 B, C. All specimens were examined alive ex- elevated at places or lies low at other parts making it a cept where indicated otherwise. somewhat backwater-like protected environment. The left margin of the ledge has much less wave action Results than the right one. In the main body of the triangular Phylum : Mollusca ledge exist small and large pools of water in which Class : Bivalvia occur cerithid snails in abundance as well as floating Subclass : Lamellibranchia algae and pelagic sea life. In the small shallow pools Order : Anisomyaria of the low tide area occurs the small larviparous oys- Family : Ostreidae ter O. nomades. This oyster also occurs on the margin Genus : Crassostrea of the pools covered with a film of silt which blends with the substratum below. The salinity here mostly in Genus: Crassostrea (Sacco) 31 the range of 35 to 40 ‰. The Keamari seawall is an Description of the genus as given by Thomson is artificial breakwater wall, about 1.5 km long, extend- as follows . “Shell elongate, very variable shape even ing into the open Arabian Sea from Karachi Harbour within the species; lower valve deep, cup-shaped, re- (Fig. 1). It is directly affected by oil pollution emanat- cessed under hinge; the muscle scar displaced both ing from the oil terminal located in Karachi Harbour. towards the dorso-lateral edge and towards the lip; Water is relatively clear and unturbid and salinities upper valve opercular, flat except sometimes upraised are generally in excess of 35 ‰ for much of the year. at the rim, may or may not have a row of denticles Paradise Point is a wave-lashed rocky shore luxuriant along the rim laterally from the hinge. A promyal in algal growth, many species of gastropds, zooanthid chamber on the right side. Non-incubatory”. and coelentartes are found here. The water is clear and the salinity ranges between 35 to 40 ‰. Saccos- trea cucullata is found blended with the substratum at a tidal height of 2-3 m. Coast of Gwadar is a T-shaped formation. Different species of gastropods, bivalves, crustaceans, coelentrates, polychaetes, echinoderms, tunicates and seaweeds are found abundantly on East and West Bays of Gwadar. The West Bay is a unique shore with four species of oysters occurring on the same boulders29. The oyster species occur at tidal height of 1.5 to 2.5 m. The oysters were measured to the nearest millime- ter by the method described by Galtsoff30 (Fig. 2A). In different sizes of Crassostrea gryphoides and C. madrasensis the hinge size was also measured from the tip to the centre of the hinge axis. The curvature of hinge to the left or right was also noted. The species Fig. 2 ⎯ A) Diagram showing the regions where height and length were measured. (After Galtsoff, 1964). B) Morphological features were identified with the help of the reports of: Awati used for the identification of oyster species, and. C) A portion of 11 18 20,21 28 & Rai , Durve , Patel & Jetani , Mane , mantle fold showing middle and inner mantle folds. 110 Indian J. Mar. Sci., Vol. 31, No. 2, June 2002 Species: Crassostrea gryphoides (Schlotheim) (Fig. 3) tion/uplifting of the oyster habitat out of water. The coast of Balochistan, in Pakistan, is well-known for Synonyms: Ostrea gryphoides Awati & Rai11, the uplifting of its terrain so that 10.26-12.82 m high Rai12, Durve17 Kazmi24, Ranjha25, Hasan26, Vreden- cliffs stand at the back of the tidal zone29 bearing a burg35; Crassostrea gryphoides Durve18, Patel & Jet- rich record of fossil marine fauna on top38. ani21, Ahmed27, Rao36 ; Crassostrea rivularis Asif28; Remarks: Earlier Asif28 reidentified this species as Crassostrea cattukensis Nagabhushanam & Mane22 . C. rivularis from Sonari (Balochistan) near Hub River In this study 685 oysters were examined from Hub Delta and indicated that the older specimen of River delta (size 12.9 – 260 mm ). Irrespective of the C. rivularis resemble the characteristics described for size of its available specimens this species showed C. gryphoides27. We find that all these oysters irre- elongated shells, anterior regions narrow and posterior spective of their sizes belonged to the species C. gry- region broad; lower valve slightly concave, heavy and phoides which were earlier identified as O. gry- thick specially below the hinge area; upper valve flat phoides (=C. gryphoides)11,17 and subsequently and less thick, forming a lid over the lower valve; C. gryphoides27. We are of the opinion that C. rivu- oysters from 41 to 240 mm size showed hinge area laris is an altogether different species, that occurs in long with a groove curving to the left in 68 % and to Japan (Numachi39) and not in Pakistan. Hence the the right in 32 % of the oysters examined; a well oyster specimens which were earlier identified as marked recess below the hinge area present in all the C. rivularis by previous authors certainly belong to specimens, the hinge area gradually increases with C. gryphoides. increase in shell size in different sizes of oysters ex- amined. Variations in shell shape in some oysters Species: Crassostrea madrasensis (Preston) (Fig. 4 A,B) were likely due to overcrowding and orientation. Promyal chamber present; colour of the inner mantle Synonyms: Ostrea cucullata Hornell4; Ostrea ma- 3 11 17 28 fold in 97 % specimens pigmented black and with two drasensis Preston , Awati & Rai , Durve , Ranjah ; rows of tentacles; in the remaining oysters it was yel- Ostrea virginiana Prashad10, Moses40; Crassostrea 18 19 21 low; tentacles of the middle mantle fold in 77 % madrasensis, Durve Rao , Patel & Jetani ; specimens were yellow and in the remaining black Crassostrea virginica, Ahmed27; Crassostrea cat- pigmented.